Patients with Obstructive Sleep Apnea (OSA) have recurrent apnoeas or hypopnoeas during sleep that are only terminated by the patient arousing. The best form of treatment for patients with OSA is constant positive airway pressure (CPAP) applied by a blower (e.g., compressor) via a connecting hose and mask. The positive pressure prevents collapse of the patient's airway during inspiration, thus preventing recurrent apnoeas or hypopnoeas and their sequelae. Such a respiratory treatment apparatus can function to supply the patient with a supply of clean breathable gas (usually air, with or without supplemental oxygen) at the therapeutic pressure or pressures, at appropriate times during the subject's breathing cycle.
Respiratory treatment apparatus typically include a flow generator, an air filter, a mask or cannula, an air delivery conduit connecting the flow generator to the mask, various sensors and a microprocessor-based controller. The flow generator may include a servo-controlled motor and an impeller. The flow generator may also include a valve capable of discharging air to atmosphere as a means for altering the pressure delivered to the patient as an alternative or in addition to motor speed control. The sensors measure, amongst other things, motor speed, gas volumetric flow rate and outlet pressure, such as with a pressure transducer, flow sensor, such as a pneumotachograph and differential pressure transducer, or the like. The apparatus may optionally include a humidifier and/or heater elements in the path of the air delivery circuit. The controller may include data storage capacity with or without integrated data retrieval/transfer and display functions.
During respiratory treatment with such a device, it is often useful to measure the subject's respiratory airflow, which may be determined with the flow sensor. However, leak between the mask and the patient are typical. Thus, the flow sensor may measure the sum of the respiratory airflow plus the flow through the leak. If the instantaneous flow through the leak is known, the respiratory airflow can be calculated by subtracting the flow through the leak from the flow at the pneumotachograph.
Known methods to correct for the flow given the leak may assume (i) that the leak is substantially constant, and (ii) that over a sufficiently long time, inspiratory and expiratory respiratory airflow will cancel. If these assumptions are met, the average flow through the flow sensor over a sufficiently long period will equal the magnitude of the leak, and the true respiratory airflow may then be calculated as described.
It is known to measure leak by calculating conductance of the leak. As described in U.S. Pat. No. 6,659,101, the conductance may be determined by dividing a low pass filtered respiratory airflow measure by a low pass filtered square root of a mask pressure measure. The instantaneous leak may then be determined by multiplying the conductance by the square root of mask pressure.
As described in U.S. Pat. No. 5,704,345 to Berthon-Jones, it is also known to determine an index of the presence of valve-like leak. The index is calculated as the ratio of the peak flow during the first 0.5 seconds of expiration to the mean flow during the second 0.5 seconds of expiration.
Another technology is disclosed in European Publication No. 0 714 670 A2, which includes a calculation of a pressure-dependent leak component. The methodology relies on knowing precisely the occurrence of the start of an inspiratory event and the start of the next inspiratory event. In other words, the leak calculation is formed as an average over a known breath and applied to a subsequent breath.
Mouth leak may still present numerous concerns to respiratory treatment therapy such as nasal CPAP therapy. Such concerns may include the following:
1. A patient's arousal rate and/or apnea and hypopnea index (“AHI”) can increase due to leak, impacting the patient's sleep architecture.
2. Ventilatory support might be reduced as a result of nasal inspiratory flow leaking out of the mouth, a particular concern for patients on Bi-level/VPAP therapy.
3. Unidirectional nasal airflow may be established, leading to dehydration of the upper airway, congestion and release of inflammatory mediators. Moreover, unidirectional nasal flow may increase nasal airway resistance, which in turn may increase the propensity for oral flow, resulting in a cycle the creates even more mouth leak
4. Patient compliance may be reduced due to nasal symptoms.
5. Erroneous behavior occurs in patient flow estimates and flow generator control algorithms, since the resulting measured total flow signal may not correctly account for oral flow.
6. Mouth breathing in children has been shown to impact their dentofacial development. Specifically, excessive oral airflow can lead to poor teeth alignment (dental malocclusion), forward head posture, irregular clavicular growth, and increased susceptibility to ear infections.
It may be desirable to develop further methods for detecting and/or measuring leak which may be implemented in respiratory treatment apparatus such as apparatus for detection and/or apparatus for treating upper respiratory conditions such as OSA.